1. Field of the Invention
The present invention relates to an optical scanner having a light beam scanning mechanism, and being mounted in an optical apparatus such as, for example, a full-color laser printer or the like.
2. Description of the Related Art
In recent years, according to a development in digital technologies, various kinds of image forming apparatuses (printers) are used as image data output apparatuses. Among them, a full-color laser printer is superior in, for example, image quality and output time.
A laser printer mainly comprises an optical system unit for image formation, that is, an optical scanner in an enclosure. In the optical scanner, when a laser beam is emitted from a light source mainly in a state in which a polygon mirror is rotating at constant speed by rotating a scan motor, the laser beam is reflected by the polygon mirror to be successively deflected, and the deflected laser beam repetitively scans a photosensitive drum. Then, an electrostatic latent image formed through scanning by the laser beam is developed by using a toner, and the developed image is transferred to a paper. Thereby, a desired image corresponding to the image data is formed on the paper. In a full-color laser printer, in order to reproduce a full-color image, for example, a light source apparatus including four light sources corresponding to four color, that is, yellow (Y), magenta (M), cyan (C), black (B) is mounted in the optical scanner, and four photosensitive drums corresponding to these four light sources are used to form an electrostatic latent image. In some cases, the polygon mirror for light deflection is contained in a closed-box-shaped containing chamber to prevent adhesion of dust, dirt or the like in the atmosphere to a reflective mirror surface.
More recently, in order to further reduce output time of the full-color laser printer, higher speed in image formation performance has been in demand. Accordingly, there has been a trend toward higher rotating speed of the scan motor.
However, when the rotating speed of the scan motor becomes higher, the amount of heat generated during rotation of the scan motor increases, thereby resulting in a temperature rise in the polygon mirror contained in the containing chamber, a motor for driving the polygon mirror, a motor drive circuit and their surroundings. When the temperature of the polygon mirror or the like rises, mainly a temperature difference in the enclosure occurs, and the enclosure is deformed resulting from the temperature difference. Scanning characteristics of the laser beam are impaired resulting from the deformed enclosure, so it is difficult to accurately form an image based upon image data, thereby resulting in degradation in image quality.
This problem is an important issue which must be overcome when a high quality full-color laser printer is designed, and in consideration of the expectation that the output time will be further reduced in future, it is necessary to prevent a temperature rise in the polygon mirror and its surroundings as much as possible, which may result in degradation in image quality.
Some specific techniques to overcome the above problem have been already proposed. For example, in Japanese Unexamined Patent Application Publication No. 2001-337290, Ono et al. disclosed a technique in which a cutting hole for thermal dissipation and a guiding path for inducing thermal dissipation are disposed in a bottom surface of an optical housing in order to prevent a temperature rise in the polygon mirror and its surroundings. However, the technique is applicable to the case where the containing chamber containing the polygon mirror is disposed in proximity to an outer surface of the optical housing, but it is difficult to apply the technique to the case where the containing chamber is disposed deep in the recesses of the optical housing.
Moreover, for example, in Japanese Unexamined Patent Application Publication No. 2001-337291, Ono et al. also disclosed a technique in which a duct for thermal dissipation is disposed on the outside of the optical housing. However, in the case of using the technique, the duct for heat dissipation is disposed, so the structure of the apparatus becomes complicated, and the manufacturing cost increases.